Add command-line and underlying API for protecting revisions which should never
be merged from accidental merging.
Requirements/use cases are here:
svnmerge.py supports the following related command set:
block Block revisions within PATH so that they disappear from the
unblock Revert the effect of 'block'
Until this is implemented, the work around is to use 'svn merge --record-only'
to block/unblock revisions from merging, and keep a separate list of revisions
which are to later be considered "blocked". It would be possible to keep such a
list in another Subversion property like "blocked-revisions" to avoid the need
for separate storage.